2001ApJ...552L.129P

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-Ray Observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum–for the pulsar magnetic field B=3x10¹² G and neutron star mass M=1.4 M_☉ and radius R^∞=13 km, we obtain T^∞_eff=0.68±0.03 MK, L^∞_bol=(2.6±0.2)x10³² ergs.s^–1, and d=210±20 pc (the effective temperature, bolometric luminosity, and radius are as measured by a distant observer). The effective temperature is lower than that predicted by standard neutron star cooling models. A standard blackbody fit gives T^∞=1.49±0.04 MK, L^∞_bol=(1.5±0.4)x10³²d²₂₅₀ ergs.s^–1 (d₂₅₀ is the distance in units of 250 pc); the blackbody temperature corresponds to a radius R^∞=(2.1±0.2)d₂₅₀ km, much smaller than realistic neutron star radii. The harder component can be modeled as a power-law spectrum, with parameters depending on the model adopted for the soft component: γ=1.5±0.3, L_X=(1.5±0.4)x10³¹d²₂₅₀ ergs.s^–1 and γ=2.7±0.4, L_X=(4.2±0.6)x10³¹d²₂₅₀ ergs.s^–1 for the hydrogen atmosphere and blackbody soft component, respectively (γ is the photon index; L_X is the luminosity in the 0.2-8 keV band). The extrapolation of the power-law component of the former fit toward lower energies matches the optical flux at γ≃1.35-1.45.